In a traditional hydraulic system for an automatic transmission clutch, fluid enters the system through a feed orifice upon actuation of a valve. The fluid flows to the clutch and strokes the clutch piston, against counteracting spring forces, to contact a clutch pack. The fluid also flows into an accumulator, against a counteracting spring force, to control the rise of pressure in the circuit. This controlled pressure rise results in a controlled rise in torque capacity in the clutch to achieve the desired shift feel.
Hydraulic circuits inherently have a delay between the time a valve is actuated to energize the circuit and the time the circuit reaches the desired pressure due to fluid and system restrictions. The automatic transmission experiences such a delay during shifting.
The shift cycle includes a delay phase and a shift phase, where the delay phase is defined as the time between valve actuation and when the clutch piston strokes to contact the clutch pack, initiating the shift. The shift phase is the development of torque capacity in the clutch thereby executing the transmission shift. The delay is a function of the flow directed to stroke the clutch piston and the displacement of the piston before contacting the clutch pack. In circuits where short delays are critical, the feed orifice is sized sufficiently large and correspondingly the accumulator must be of a larger size to accommodate the larger volume of fluid. High flow rates and large accumulator volumes have disadvantages such as greater pressure drops across circuit restrictions and greater packaging requirements, respectively.
Another way to reduce shift delay is by increasing feed flow by introducing high pressure pulses for short time periods or using pressure pulses to open a valve to bypass the restrictive feed orifice when a feed pressure level is exceeded. Such a system requires high pressures that may be difficult to generate. Further, pressure control requires complex, adaptive algorithms to ensure proper timing of the pulse relative to the end of the delay phase.
In reducing the delay phase, one must also consider the requirements of the shift phase. Once the shift begins, the rate of increase in the system pressure must be controlled. If the rate is too great, then clutch pressure and therefore clutch torque capacity will increase too quickly, resulting in a hard-feeling shift. Likewise if the rate of increasing clutch torque capacity is too slow, then the shift may feel as though it is dragging.